This invention relates to thermosettable resin compositions. The invention further relates to a new class of homopolymerizable and copolymerizable monomers. In one aspect, the invention relates to methods for preparing new polymerizable monomers.
Thermosettable resins are used in applications, such as advanced aerospace composites, requiring good high-temperature properties, including high (above 200.degree. C.) cured glass transition temperature and toughness, as reflected in Mode I fracture toughness values above 2 MPa-m.sup.1/2. Examples of thermosettable resins useful in advanced composites include epoxy resins and bismaleimide resins. Epoxy resins have good processing properties but relatively low glass transition temperatures. Bismaleimide resins have superior high-temperature properties but are very brittle. Standard toughening modifiers for bismaleimides, such as diallylbisphenol-A, generally do not give fracture toughness values greater than about 1 MPa-m.sup.1/2 when copolymerized with bismaleimides, and attaining even this degree of toughness often requires the use of so much modifier that the Tg of the cured product is lowered considerably.
Increasingly important applications for thermosettable resins are electrical laminates and encapsulation, for which low dielectric constant is an additional performance requirement. In general, monomers in which the polymerizing group is hydrocarbon, such as butadiene oligomers and mono- and difunctional allyl monomers, provide low dielectric constant polymers, but polymerization of such monomers is slow, requires large quantities of free-radical initiators such as peroxides (which produce contaminating residues), and produces materials having low toughness relative to their heat resistance.
Bisbenzocyclobutene monomers such as those described in U.S. Pat. No. 4,540,763 homopolymerize to cured materials having low dielectric constant and high Tg. When these monomers are copolymerized with bismaleimides, as described in U.S. Pat. No. 4,973,636, it is possible to achieve cured materials having a Mode I fracture toughness of almost 5 MPa-m.sup.1/2. However such bisbenzocyclobutene monomers are quite expensive to prepare, requiring multistep syntheses from the nearest commercially-available material. The cost of the monomer precludes these polymers from practical application in most end-uses.
It is thus an object of the invention to provide new thermoset resin materials. In one aspect, it is an object of the invention to provide monomers which polymerize to low dielectric constant materials. In a further aspect, it is an object of the invention to provide relatively inexpensive comonomers for bismaleimides which improve the toughness of the bismaleimides without unduly sacrificing Tg.